There has been a disadvantage with FM-CW radars using millimeter radio waves as car-mounted radars in that FM-CW radars interfere with radars mounted on other cars. That is, as shown in part (A) of FIG. 4, when a subject car MM on which a radar of a type in which beams are scanned in an azimuthal direction is mounted faces an object car OM1 of the same type, spike noise is superimposed on (mixed with) a beat signal at the timing of generating a beat of a transmission signal directly received from the object car OM1 and a transmission signal of the subject car MM. Moreover, when a subject car MM on which a radar of a type in which beam scanning is performed as shown in part (B) of FIG. 4 is mounted faces an object car OM2 on which a monopulse radar is mounted, spike noise is also superimposed on a beat signal of a transmission signal and a reception signal. Furthermore, as shown in part (C) of FIG. 4, if an object car OM4 traveling ahead of a subject car MM exists and an object car OM3 transmitting a radio wave toward this object car OM4 exists, a signal which is transmitted from a radar mounted on the object car OM3 and reflected at the object car OM4 is superimposed on a reception signal of the subject car MM, and thereby spike noise is also superimposed on a beat signal.
Patent Document 1 discloses a method for detecting such spike noise.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-160512.
A method disclosed in Patent Document 1 includes obtaining a frequency spectrum of a beat signal by an FFT, detecting the presence or absence of an interference using the presence or absence of an increase in a noise floor of the frequency spectrum, and changing a transmission frequency in the case of detecting the presence of interference. This transmission frequency is repeatedly changed until no interference becomes detected (until the influence caused by the interference becomes small).
However, an FM-CW radar needs a wide frequency band in order to perform frequency modulation; therefore, changing a frequency is substantially difficult because of oscillator characteristics or legislation. In addition, targets cannot be detected while a frequency is being changed. Furthermore, because, for example, a cycle period of the beam scanning in an azimuthal direction becomes inconstant due to a change in a frequency, problems may occur in functions such as examining the correlation between frequency spectra of the beat signals of beams in the same direction, assuming targets having a strong correlation as the same target, and thereby tracking the target. That is, target tracking is interrupted every time a frequency is changed.